The present invention relates to a method for manufacturing an incandescent lamp.
A conventional incandescent lamp, for example, a tungsten halogen lamp which lights up with a commercial voltage comprises, as shown in the front view of FIG. 8 and the side view of FIG. 9, a bulb 11 having an overall length of 55 mm. The bulb 11 has a spheroidal expanded portion 11a, a narrowed portion 11b and a pinch-sealed portion 11c. The bulb 11 contains inside thereof a filament assembly 12 comprised of a filament coil. The maximum outside diameter of the expanded portion 11a is 14 mm.
A method for manufacturing such a tungsten halogen lamp is as follows.
As shown in FIG. 10, a cylindrical glass bulb 14 communicating at one end thereof with an exhaust tube 13 is heated to be softened by a burner (not shown). Afterwards, while closing the end opposite to the exhaust tube 13 of the glass bulb 14 with a closing member 15 and charging a protective gas from the exhaust tube 13, dies (not shown) are pressed to the end on the exhaust tube 13 side of the glass bulb 14, thereby forming the expanded portion 11a and the cylindrical narrowed portion 11b. 
Then, as shown in FIG. 11, the closing member 15 is detached and the filament assembly 12 is disposed inside the glass bulb 14. A cylindrical portion 14a opposite to the exhaust tube 13 of the glass bulb 14 is heated to be softened by a burner 16, and as shown in FIG. 12, the cylindrical portion 14a is pinch-sealed by dies 17 having an L shape in cross section, thereby forming the pinch-sealed portion 11c as shown in FIG. 13.
The glass bulb 14 is pumped through the exhaust tube 13 and is filled with a predetermined amount of a filling gas, and the exhaust tube 13 is sealed. In this manner, a tungsten halogen lamp is manufactured.
Such a conventional method for manufacturing the tungsten halogen lamp does not pose a problem when it is applied to a lamp which burns with a commercial voltage. In the case of manufacturing a smaller tungsten halogen lamp which burns with a lower voltage of, for example, about 12V or 24V, however, the following problems arise. When the expanded portion 11a and the narrowed portion 11b are formed in advance and then the pinch-sealed portion 11c is formed, due to the effects of heat of the burner 16 at the time of pinch-sealing and of the pinch-sealing itself, the expanded portion 11a and the narrowed portion 11b are distorted, and the commercial value of the lamp deteriorates.
The distortion of the expanded portion 1 la and the narrowed portion 11b causes the thickness of each of the expanded portion 11a and the narrowed portion 11b to vary. As a result, a problem arises that the bulb 11 is broken due to the pressure of the filling gas (higher than 1 atmospheric pressure) when the lamp is burned.
Further, it is necessary to close the end opposite to the exhaust tube 13 of the glass bulb 14 with the closing member 15 in order to form the expanded portion 11a and the narrowed portion 11b. It causes a problem that the number of manufacturing steps increases and the cost becomes higher. The present invention has been achieved to solve these problems and has an object to provide a method for manufacturing an inexpensive lamp capable of preventing distortion of the bulb, decrease in the commercial value and breakage of the bulb.
According to the invention, there is provided a method for manufacturing a lamp comprising a bulb containing therein a filament assembly and having an expanded portion and a pinch-sealed portion, the method comprising the steps of: softening a bulb communicating at one end thereof with an exhaust tube, with the filament assembly disposed inside the bulb; pinch-sealing the end opposite to the exhaust tube of the bulb; and then forming the expanded portion by pressing dies each having a predetermined shape to the bulb by charging a protective gas from the exhaust tube.
Consequently, distortion of the bulb can be prevented and the manufacturing process can be simplified.